It is know that bypass turbojet engines comprise a nacelle defining an air inlet at the front and axially containing a cold stream fan, a central hot stream generator and a fan duct of annular section provided, at the rear, with a jet pipe nozzle for said cold stream, and that, in at least some of these turbojet engines:                said cold stream jet pipe nozzle is formed by an outer fan cowl and by an inner fan cowl of which the initial rear parts are respectively convex and concave and converge toward one another until they meet to form an initial outlet orifice for the cold stream;        a sound deadening coating of annular section that has to have a preset optimum thickness in order effectively to deaden the noise generated by said fan and carried along in said cold stream, said coating being borne internally by said inner fan cowl at the location where the distance between said converging parts of said inner and outer fan cowls is at least equal to said optimal thickness of the sound deadening coating;        said hot stream generator is enclosed in an axial engine cowl that has at least approximately the shape of a divergent front conical surface and of a convergent rear conical surface opposing one another on a common base which lies forward of said initial cold stream outlet orifice, the initial jet pipe nozzle throat and the initial cold stream outlet section being delimited between the initial rear part of the inner fan cowl and the rear conical surface of said engine cowl, said rear conical surface comprising, in its rear part, at least one opening which is positioned on the outside with respect to said cold stream initial outlet orifice and which is intended to discharge to the outside a stream of ventilating air bled from said cold stream and introduced into said engine cowl to regulate the temperature of said hot stream generator; and        said fan duct is delimited between said inner fan duct and said engine cowl.        
In a turbojet engine such as this, the rear part of the cold stream jet pipe nozzle may have noise-deadening characteristics that are not optimal because throughout that part of it in which the distance between the converging rear parts of said inner and outer fan cowls is smaller than said optimal thickness of said noise deadening coating, there is no space to house said coating.